E106 Distinct synovial fluid cell profiles are associated with different types of inflammatory arthritis

Background/Aims A key feature of inflammatory arthritis is the development of synovial inflammation and joint effusions. These effusions are often amenable to diagnostic arthrocentesis, and the synovial fluid can be analysed in routine clinical practice with standard microbiology laboratory practices. It is accepted that a raised total white blood cell (WBC) count typically indicates inflammatory arthritis (IA); however, it is not clear whether different types of IA or treatment with disease-modifying antirheumatic drugs (DMARDs) are associated with distinct synovial fluid cell profiles. Methods Routine clinical data and synovial fluid cytology results were retrospectively reviewed for 367 patients who underwent diagnostic arthrocentesis from 2022-2024 in Greater Manchester with samples processed by the Manchester Cytology Centre. Differences between groups were assessed using independent t-tests, chi-square, or Mann-Whitney U tests as appropriate. Statistical analyses were performed in R version 4.4.3. Results Higher WBC counts and higher proportions of lymphocytes, neutrophils, macrophages, and synoviocytes were found in patients with a rheumatological diagnosis compared to a non-rheumatological diagnosis. When comparing patients with rheumatoid arthritis (RA) and those with a spondyloarthropathy (SpA) (psoriatic arthritis, enteropathic arthritis, or axial spondyloarthritis), lower WBC counts and presence of cytophagocytic macrophages (CPMs) were found in patients with SpA compared to RA. Higher WBC counts were found in patients with crystal arthropathies compared to those with other forms of IA, but higher proportions of lymphocytes and macrophages were found in IA compared to crystal arthropathies. No differences in synovial fluid cell profiles were found between patients with IA receiving or not receiving conventional synthetic or biologic DMARDs. Conclusion Distinct synovial fluid cell profiles were found in different types of IA, which may be helpful in the diagnosis of different types of IA and in the interpretation of synovial fluid analysis results in routine clinical practice. Disclosure F. Oram: None. E. Colman: None. C. Ciuculete: None. K. Santilal: None. S. Collis: None. P. Kamath: None. P. Ho: None. A. Barton: None. R.M. Hum: None.